1. Field of the Invention
This invention relates to a method of double decomposition of a salt. More specifically, it relates to a simple and convenient method by which a salt typified by sodium chloride, generally a neutral salt, is decomposed by using a three-compartment electrodialysis cell comprising a special ion-exchange membrane to form an acid and an alkali in high concentrations at a high current efficiency.
2. Description of the Prior Art
Methods involving the use of an ion-exchange membrane have previously been known for the formation of an acid such as hydrochloric acid and an alkali such as sodium hydroxide by double decomposition of a salt such as sodium chloride. Such a method is carried out in a three-compartment electrodialysis cell comprised of an anode, a cathode, sets of a cation-exchange membrane, a bipolar membrane (composite membrane) and anion-exchange membrane sequentially and repeatedly arranged between the anode and the cathode, a salt compartment formed between the cation-exchange membrane and the anion-exchange membrane, a base compartment formed between the cation-exchange membrane and the anion-exchange resin side of the bipolar membrane and an acid compartment between the cation-exchange resin side of the bipolar membrane and the anion-exchange membrane. By applying a direct-current voltage across the anode and the cathode of the three-compartment ion-exchange membrane electrodialysis cell while passing an aqueous solution of a salt through the salt compartment, a dilute alkali solution or water through the base compartment and dilute hydrochloric acid or water through the acid compartment, ions of the salt, for example a sodium ion (Na.sup.+) and a chlorine ion (Cl.sup.-) of sodium chloride, permeate the cation-exchange membrane and the anion-exchange membrane respectively and are discharged in the base compartment and the acid compartment. At the same time, water dissociated in the bipolar membrane is separated into a hydroxyl ion (OH.sup.-) and a hydrogen ion (H.sup.+), whereby sodium hydroxide (NaOH) is formed in the base compartment and hydrochloric acid (HCl), in the acid compartment.
The method in which a salt is double-decomposed in a conventional three-compartment electrodialysis cell, however, has the disadvantage that the concentrations of the resulting acid and alkali and the current efficiency are low. This is because the acid and alkali diffuse through the bipolar membrane and neutralization occurs, or the permeability of the hydrogen ion (H.sup.+) through the anion-exchange membrane is high, and/or the permeability of the hydroxyl ion (OH.sup.-) through the cation-exchange membrane is high. For example, when ordinary cation-exchange membranes, bipolar membranes and anion-exchange membranes are used, the concentrations of the resulting acid and alkali is several percent by weight, and the current efficiency is about 70%, and attempts to increase their concentrations result in a rapid decrease in current efficiency.